DaisyDisk - DaisyDisk is a disk analyzer tool for OS X that visualizes hard disk usage and allows to free up hard disk space.
WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
CleanMyMac X - Clean, protect and speed up your Mac
TreeSize - TreeSize tells you where precious disk space has gone to.
MacFly Pro - Smart assistant for better Mac performance
SpaceSniffer - SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks.